1. Field of the Invention
The present invention relates to measuring optical characteristics, such as chromatic dispersion characteristics of a DUT (Device Under Test) such as a fiber pair, and more specifically relays to measuring chromatic dispersion characteristics where independent measuring machines are connected tooth ends of the DUT.
2. Description of the Related Art
When light is transmitted over a long distance, losses are large if the light is transmitted only through an optical fiber. The loss can be prevented by using an optical fiber line where an optical fiber is combined with an optical amplifier (EDFA), which amplifies a light signal. The light amplifier allows light to pass in only a certain direction. For bi-directional communication, one optical fiber line for transmitting light in one direction, and another optical fiber line for transmitting light in a direction opposite to that one direction are combined as a cable. This cable is called one fiber pair.
FIG. 6(a) shows a constitution of one fiber pair. An optical fiber line 110 is formed by combining an optical fiber 112 with optical amplifiers 114. The optical fiber line 110 passes light in the right direction of FIG. 6(a). The optical fiber line 120 is formed by combining an optical fiber 122 with optical amplifiers 124. The optical fiber line 120 passes light in the left direction of FIG. 6(a). The optical fiber line 110 and the optical fiber line 120 form one fiber pair 100a. Two fiber pairs are shown in FIG. 6(b). Fiber pairs 100a and 100b form two fiber pairs 100.
FIG. 7 shows a constitution of a measuring system when chromatic dispersion characteristics of two fiber pairs are measured. A variable wavelength light source 202 is connected to one end, and an O/E (optical/electrical) converter 302 is connected to the other end of one fiber pair 100a included in the two fiber pairs 100. A fixed wavelength light source 204 is connected to one end, and an O/E (optical/electrical) converter 304 is connected to the other end of one fiber pair 100b included in the two fiber pairs 100. Optical modulators may be provided between the variable wavelength light source 202 and the fiber pair 100a and/or between the fixed wavelength light source 204 and the fiber pair 100b. 
For measuring the chromatic dispersion characteristic, the wavelength xcexx of variable wavelength light source 202 is swept (changed at a constant speed) while the wavelength xcex0 of the fixed wavelength light source 204 is fixed. A phase comparator 306 measures a phase difference between an output signal from the O/E converter 302 and an output signal from the O/E converter 304, thereby measuring the wave dispersion characteristic of two fiber pairs.
In a high capacity transmission line in a trunk line system, it may be possible to use two fiber pairs. However, only one fiber pair can be used in most of the lines which have already been laid down. Thus it is necessary to measure the chromatic dispersion characteristic of one fiber pair.
The measuring method for the chromatic dispersion characteristic described above does not apply to one fiber pair. This is because two lines which pass light in the same direction and comprise a line for passing the fixed wavelength light and a line for passing the variable wavelength light are not available in one fiber pair.
When the measuring method for the chromatic dispersion characteristic described above is applied to the two fiber pairs 100, an error may be generated in a measured result. A difference between phases of light transmitted through the one fiber pair 100a and the one fiber pair 100b may change due to components which do not depend on wavelength when physical changes such as a temperature change or a stress change in the transmission line occur. In these cases, an error is generated in a measured result. It is desirable to use only one fiber pair instead of using two fiber pairs for measuring the chromatic dispersion characteristic.
The purpose of present invention is to provide an apparatus and the like for measuring the chromatic dispersion characteristic and the like with only one fiber pair.
According to the present invention, an optical characteristics measuring apparatus for measuring characteristics of light transmitted through a device under test may include: a variable wavelength light source for generating variable wavelength light, whose wavelength is variable a fixed wavelength light source for generating fixed wavelength light, whose wavelength is fixed a light modulating unit for modulating the variable wavelength light and the fixed wavelength light with a predetermined frequency a composite light generating unit for entering composite light composed of the variable wavelength light and the fixed wavelength light into the device under test a fixed wavelength component extracting unit for extracting the fixed wavelength light component from transmitted light, which is transmitted through the device under test and a variable wavelength component extracting unit for extracting the variable wavelength light component from the transmitted light.
According to the optical characteristics measuring apparatus constituted as described above, the fixed wavelength component extracting unit extracts the fixed wavelength light component from transmitted light transmitted through a device under test, and the variable wavelength component extracting unit extracts the variable wavelength light component from the transmitted light transmitted through the device under test. Thus, a phase difference of the variable wavelength light component is measured with the fixed wavelength light component as a reference. Since the phase difference is measured with one line for passing composite light, the measuring is possible when a device under test is one fiber pair.
According to the present invention, an optical characteristics measuring apparatus for measuring characteristics of light transmitted through a device under test includes: a variable wavelength light source for generating variable wavelength light, whose wavelength is variable a fixed wavelength light source for generating fixed wavelength light, whose wavelength is fixed a light modulating unit for modulating the variable wavelength light and the fixed wavelength light with a predetermined frequency and a composite light generating unit for entering composite incident light composed of the variable wavelength light and the fixed wavelength light into the device under test.
According to the present invention, an optical characteristics measuring apparatus for measuring characteristics of light transmitting device under test may include: a fixed wavelength component extracting unit for extracting a fixed wavelength light component from transmitted light, which is generated after light, which is composed of variable wavelength light whose wavelength is variable and fixed wavelength light whose wavelength is fixed, has been transmitted through the device under test and a variable wavelength component extracting unit for extracting the variable wavelength light component from the transmitted light.
In the present invention as described above the fixed wavelength light source may switch the wavelength of the fixed wavelength light.
In the present invention as described above, wherein the composite light generating unit may be a coupler.
In the present invention as described above, the variable wavelength component extracting unit may be an optical filter, which reflects the fixed wavelength light component, and transmits the variable wavelength light component, and the fixed wavelength component extracting unit may be a circulator, which has a first terminal for receiving the transmitted light, a second terminal for outputting the light received by the first terminal and for receiving inputted light, and a third terminal for outputting the light received by the second terminal, and wherein the second terminal is connected to the optical filter.
In the present invention as described above, the fixed wavelength light source may be provided with: a first fixed wavelength light source for generating first fixed wavelength light, a second fixed wavelength light source for generating second fixed wavelength light, an output terminal, and a switch for connecting the output terminal with either the first fixed wavelength light source or the second fixed wavelength light source.
In the present invention as described above, the device under test may have a first light path for passing light in only one direction, and a second light path for passing light in only one direction opposite to the one direction.
In the present invention as described above, the device under test may have a first light path for passing light in only one direction, and a second light path for passing light in only one direction opposite to the one direction, the variable wavelength light source, the fixed wavelength light source, the light modulating unit, and the composite light generating unit being connected to an entrance side, of the first light path, and the fixed wavelength component extracting unit and the variable wavelength component extracting unit being connected to an exit side of the second light path.
The present invention above, may further include: an optical/electrical converting unit for converting the fixed wavelength light component and the variable wavelength light component into electric signals a phase comparing unit for measuring a phase difference between the electric signal corresponding to the fixed wavelength light component, and the electric signal corresponding to the variable wavelength light component and a characteristics calculating unit for calculating group delay characteristic or dispersion characteristic of the device under test by using the phase difference.
According to the present invention, an optical characteristics measuring method for measuring characteristics of light transmitted through a device under test may include: a variable wavelength light generating step for generating variable wavelength light, whose wavelength is variable a fixed wavelength light generating step for generating fixed wavelength light, whose wavelength is fixed a light modulating step for modulating the variable wavelength light and the fixed wavelength light with a predetermined frequency a composite light generating step for entering composite light composed of the variable wavelength light and the fixed wavelength light into the device under test a fixed wavelength component extracting step for extracting the fixed wavelength light component from transmitted light, which is transmitted through the device under test and a variable wavelength component extracting step for extracting the variable wavelength light component from the transmitted light.
According to the present invention, an optical characteristics measuring method for measuring characteristics of light transmitted through a device under test may include: a variable wavelength light generating step for generating variable wavelength light, whose wavelength is variable a fixed wavelength light generating step for generating fixed wavelength light, whose wavelength is fixed a light modulating step for modulating the variable wavelength light and the fixed wavelength light with a predetermined frequency and a composite light generating step for entering composite incident light composed of the variable wavelength light and the fixed wavelength light into the device under test.
According to the present invention, an optical characteristics measuring method for measuring characteristics of light transmitting device under test may include: a fixed wavelength component extracting step for extracting a fixed wavelength light component from transmitted light, which is generated after light, which is composed of variable wavelength light whose wavelength is variable and fixed wavelength light whose wavelength is fixed, has been transmitted through the device under test; and a variable wavelength component extracting step for extracting the variable wavelength light component from the transmitted light.
The present invention may also be a computer-readable medium having a program of instructions for execution by the computer to perform an optical characteristics measuring process for measuring characteristics of light transmitted through a device under test, the optical characteristics measuring process including: a variable wavelength light generating processing for generating variable wavelength light, whose wavelength is variable a fixed wavelength light generating processing for generating fixed wavelength light, whose wavelength is fixed a light modulating processing for modulating the variable wavelength light and the fixed wavelength light with a predetermined frequency a composite light generating processing for entering composite light composed of the variable wavelength light and the fixed wavelength light into the device under test a fixed wavelength component extracting processing for extracting the fixed wavelength light component from transmitted light, which is transmitted through the device under test and a variable wavelength component extracting processing for extracting the variable wavelength light component from the transmitted light.
The present invention may be a computer-readable medium having a program of instructions for execution by the computer to perform an optical characteristics measuring process for measuring characteristics of light transmitted through a device under test, the optical characteristics measuring process including: a variable wavelength light generating processing for generating variable wavelength light, whose wavelength is variable a fixed wavelength light generating processing for generating fixed wavelength light, whose wavelength is fixed a light modulating processing for modulating the variable wavelength light and the fixed wavelength light with a predetermined frequency and a composite light generating processing for entering composite incident light composed of the variable wavelength light and the fixed wavelength light into the device under test.
The present invention may be a computer-readable medium having a program of instructions for execution by the computer to perform an optical characteristics measuring process for measuring characteristics of light transmitted through a device under test, the optical characteristics measuring process including: a fixed wavelength component extracting processing for extracting a fixed wavelength light component from transmitted light, which is generated after light, which is composed of variable wavelength light whose wavelength is variable and fixed wavelength light whose wavelength is fixed, has been transmitted through the device under test and a variable wavelength component extracting processing for extracting the variable wavelength light component from the transmitted light.